1. Field of the Invention
The present invention generally relates to a packet transmission control device and a packet transmission control method, and more particularly to a packet transmission control device and a packet transmission control method for controlling transmission (or scheduling) of downlink packets in a mobile communication system.
2. Description of the Related Art
In the downlink of the mobile communication system, a wireless base station may share one single physical channel with mobile terminals, or mobile stations, that belong to the base station. The physical channel used in such a case will hereinafter be referred to as a downlink shared channel.
In the downlink shared channel, the wireless base station may increase the throughput, or so-called system capacity, by controlling the order of packets to be transmitted to multiple mobile stations that are communicating parties, based on the radio channel quality of each mobile station in an instant. It is known that such control of the order of packets to be transmitted by the wireless base station, which is referred to as “scheduling,” may be applied to packet data transmission to increase communication capacity or improve the communication quality (for example, see J. M. Holtzman, IEEE VTC2000 spring). In conventional scheduling, it has been generally assumed that packets to be scheduled should not have severe requirements on transmission delay.
In standardizing the third generation mobile communication system, or so-called IMT-2000, 3GPP and 3GPP2 (Third-Generation Partnership Project/Third-Generation Partnership Project 2) are working on establishing standard specifications, and those associated with W-CDMA are directed by the 3GPP and those associated with cdma2000 by the 3GPP2.
With the rapid growth of the Internet in recent years, standardization of “HSDPA” (High-Speed Downlink Packet Access), which is a high-speed packet transmission mode in the downlink direction, is under way in the 3GPP based on the assumption that, especially in the downlink, a high-speed large volume of traffic will grow due to, for example, downloads from databases and Web sites (for example, see 3GPP TR25.848 v4.0.0) The 3GPP2 is also conducting the standardization of “1x EV-DO” which is a transmission mode dedicated to high-speed data in the downlink direction from the same point of view as above (for example, see 3GPP2 C. S0024 Rev.1.0.0). The term “DO” in cdma2000 1x EV-DO refers to “Data Only.”
The HSDPA, for example, is adapted to increase throughput on each individual user and overall system throughput by using a scheme, which is for example referred to as “AMCS” (Adaptive Modulation and Coding Scheme) in the HSDPA, for controlling modulation or encoding rate for a wireless channel depending on radio channel conditions between a mobile station and a wireless base station, in conjunction with scheduling operative in some milliseconds cycles.
A round-robin scheduler is well known for a scheduling algorism for controlling the order in which queued packets in the wireless base station are to be transmitted. The round-robin scheduler controls the order of packets to be transmitted by assigning the downlink shared channel sequentially (for example, mobile station #1 to # 2 to #3 . . . ) to mobile stations belonging to the wireless base station.
The Proportional Fairness scheduler and MAX C/I (Maximum C/I) scheduler are also known, and they control the order in which queued packets are to be transmitted based on the radio channel condition for each mobile station or an average transmission rate for each mobile station. An example of the typical Proportional Fairness scheduler will now be described on how it effectuates control.
The Proportional Fairness scheduler is a scheduling algorism that performs transmission assignment depending on instantaneous variation in downlink radio channel conditions for each individual mobile station and also supports fairness between mobile stations. Now, the Proportional Fairness scheduler will briefly be described. FIG. 10 is a flowchart illustrating operation of the Proportional Fairness scheduler. The scheduling algorism measures radio channel conditions and an average transmission rate for each mobile station as elements for an evaluation function, determines the evaluation function for each mobile station belonging to the wireless base station, and then assigns a shared channel to a mobile station that has the maximum evaluation function value.
In the figure, an initial value is set as shown below at step S41:
(Initial Value)
n=1*(n: an index for a mobile station),
Cmax=0 (Cmax: the maximum value from the evaluation function),
nmax=0 (nmax: an index for a mobile station that has the maximum evaluation function value).
At step S42, necessary elements for calculation of the evaluation function, specifically (1) an instantaneous radio channel condition Rn for each mobile station, and (2) an average transmission rate avrgRn are measured. At step S43, the evaluation function Cn, which is based on the following equation, is calculated using values of the above (1) and (2) measured at step S42:Cn=Rn/avrgRn 
At step S44, it is determined whether the evaluation function value Cn calculated at step S43 exceeds Cmax. In this case, Cmax=0, and then it is determined to be YES at step S44, and at step S45, the value of Cn calculated at step S43 is set to Cmax, or nmax=1. Then at step S46, n is incremented by +1, the evaluation function values are sequentially determined by the number of mobile stations that are in communication with the wireless base station through the loop procedure at step S47, and at step S48, a mobile station that has the maximum evaluation function value is selected and the shared channel is assigned to the mobile station.
The Proportional Fairness scheduler is expected to provide higher throughput than the round-robin scheduler because the transmission assignment is performed under a relatively good condition in downlink channel quality for each mobile station. In addition, using division by the average transmission rate for each mobile station may reduce the evaluation function value of a mobile station that has a higher average transmission rate to provide highly fair assignment in terms of time.
Here, because the evaluation function according to the Proportional Fairness scheduler calculates in such a way that the numerator, which is the instantaneous radio channel condition Rn, is divided by the denominator, which is the average transmission rate avrgRn, the evaluation function value Cn will become very large as the average transmission rate avrgRn approaches zero. In this case, this implies that the average transmission rate avrgRn is equal to zero, that is, the frequency of assigning a shared channel to the mobile station n is low, and it is generally proper operation that the evaluation function value Cn becomes large because a shared channel should be assigned to the mobile station n. In mobile communication, however, a mobile station may abruptly enter a tunnel or basement where radio waves cannot reach the station, and therefore the average transmission rate avrgRn may essentially approach zero. The term “essentially” here means that the average transmission rate would approach zero even when ample shared channel is assigned to the mobile station. In such a case, the shared channel must be assigned to other mobile stations in addition to the mobile station n in terms of the entire system. In other words, a phenomenon of the evaluation function value getting larger for such mobile station in an abnormal state as described above poses a problem of performance of the entire system being degraded.
On the other hand, high-speed packet transmission modes such as the HSDPA and 1x EV-DO generally employ a best effort type communication, and they provide a data transmission with a higher communication speed when there are small number of mobile stations to which the channel is assigned, and provide a data transmission with a lower communication speed when there are large number of mobile stations to which the channel is assigned. For such high-speed packet transmission modes, however, it is under consideration to provide services such as Streaming and VoIP, which are required to satisfy predetermined requirements on transmission delay. Therefore, it is required to provide a Guaranteed Bit Rate that guarantees a certain minimum rate for predetermined service types in the high-speed packet transmission modes. The Guaranteed Bit Rate implies a control in which, for example, a minimum of 64 kbps is guaranteed for any mobile stations that are using Streaming.
The following approach, for example, is conceivable as a method for providing the Guaranteed Bit Rate feature in the Proportional Fairness scheduler: for the evaluation function Cn, Cn=Rn/(avrgRn−Rtarget) may be used in place of Cn=Rn/avrgRn, where Rtarget is the transmission rate to be guaranteed.
However, even with the Proportional Fairness scheduler having the Guaranteed Bit Rate feature, there is a problem that the average transmission rate avrgRn approaches the transmission rate to be guaranteed Rtarget, or the evaluation function value Cn becomes extremely large as the average transmission rate avrgRn falls below the transmission rate to be guaranteed Rtarget.
JP2002-261772A describes a technique in which, if wireless stations that use different wireless transmission rates to transmit packets coexist in the same wireless channel, the wireless stations are grouped by wireless transmission rates depending on the traffic condition and packets are assigned to wireless channels that are different for each group to maintain good transmission efficiency.
Further, JP2003-152630A describes a technique in which, in the transmission assignment of the shared channel, temporal distribution of instantaneous downlink quality information is added to decision factors in addition to instantaneous downlink quality and average downlink quality for each mobile station to perform the transmission assignment of the shared channel for temporally fair transmission assignment.
As described above, the Proportional Fairness scheduling is one of scheduling algorisms for determining the order in which queued packets in the wireless base station are to be transmitted.
However, the conventional Proportional Fairness scheduling algorism suffers from a problem of degraded performance of the entire system because, if a mobile station transitions to an abnormal state, the average transmission rate avrgRn which is a denominator of the evaluation function approaches zero, and as a result, the evaluation function value Cn becomes very large and an unnecessarily large number of shared channels are assigned to the mobile station.
The problem described above also resides not only in the conventional Proportional Fairness scheduling algorism but also in the scheduling algorism that provides the Guaranteed Bit Rate feature as described above.
Furthermore, such a problem cannot be solved with techniques in JP2002-261772A or JP2003-152630A described above.
The present invention has been made in view of the above problem, and it is an object of the invention to provide a packet transmission control device and a packet transmission control method for possibly reducing degradation of system throughput due to a mobile station in an abnormal state.